1. Field of the Invention
This invention relates to the field of hematology and, more particularly, to methods and apparatus for preparing blood samples for cell analysis. More specifically, it relates to improvements in methods and apparatus for gently mixing whole blood with diluents and/or reagents (e.g. lytic reagents and stains) to facilitate the detection, differentiation and counting of different cell types in a whole blood sample.
2. Discussion of Prior Art
A thorough analysis of a whole blood sample is commonly effected by examining both the chemical composition of the blood and its cellular makeup. In analyzing the chemistry of blood, a blood sample is first “spun down” with a centrifuge or the like, thereby physically separating the blood cells from the serum in which the cells are normally suspended. The serum, which is the subject of a blood chemistry analysis, is then delivered to an automated clinical chemistry analyzer. The latter initially operates to dispense relatively small volumes of the serum into a plurality of open cuvettes that either already contain, or shortly thereafter receive, specific reagents that facilitate the detection of a particular chemical element, compound, enzyme, etc., of interest in the blood sample. Prior to being analyzed for a characteristic of interest, the blood sample and reagent are thoroughly mixed together to provide a homogeneous mixture, as required for the chemical analysis. For several reasons, not the least of which is to provide a relatively fast cycle time, the mixing process is usually vigorous, sometimes being effected by shooting the reagent liquid into the serum-containing cuvette under pressure, or by using ultrasonic vibrating techniques. The former approach is useful when a relatively large volume of reagent is to be mixed with the blood sample. An alternative mixing scheme is disclosed in U.S. Pat. No. 4,815,978 to Mazza et al. where mixing biological liquids (including blood serum) with reagents and/or diluents in an open-mouth cuvette is achieved by an air jet that operates from a distance to direct a stream of pressurized air at the liquid surface within the cuvette. To achieve optimum mixing, the air jet is directed at a point adjacent the junction of the liquid surface and the cuvette wall. Further, the air jet is inclined so that the angle of incidence made by the air stream and the liquid surface is relatively shallow, whereby a vortex is produced at which the different materials to be mixed are caused to converge and thoroughly mix together. The patent disclosure notes the desirability of maximizing the horizontal component of the air jet, i.e., inclining the air jet more nearly to the liquid surface. In a preferred embodiment, the acute angle at which the air jet attacks the liquid surface is between 8 and 15 degrees measured with respect to horizontal, i.e., the surface of the liquid within the cuvette.
In the field of hematology where the cellular make-up of a blood sample is determined, the process of preparing the sample for analysis is considerably, and necessarily, different from that used in clinical chemistry. In a hematology instrument, the whole blood sample itself is prepared for analysis, not merely the serum. In mixing the whole blood sample with the diluents and/or reagents by which the various different cells can be readily differentiated from other cell types in the sample, it is paramount that the integrity, and especially the morphology, of the cells be preserved. By preserving the integrity of the cells, each cell can be readily distinguishable from other types of cells in the sample on the basis of its physical size (volume), its light-reflecting characteristics, its RF conductivity, and any other parameter by which blood cells can be differentiated from other cell types in the sample. This requirement of preserving the cell integrity dictates a much more gentle mixing process than that used, or proposed for use, in mixing the sample serum with reagents in blood chemistry analyzers. A gentle mixing is especially important when the blood sample is mixed with relatively aggressive reagents, such as lytic reagents, that operate to attack and eliminate certain types of cells (e.g., red cells) so that other cells (e.g., white cells) can be more easily detected and counted. Thus, in hematology instruments designed to automatically differentiate and enumerate various different cell types in a blood sample, it is common employ motor-driven devices, such as orbital mixers and rotating paddles to impart a relatively slow circular motion to the sample-containing reaction vessel, a motion that mimics the gentle orbital mixing movement that can be provided by the human hand. Alternatively, motor-driven rockers or vibrators have been used to gently mix blood samples hematological analysis. While such conventional mixing devices have proven highly effective in producing good sample mixing with minimum damage or alteration to the blood cells, such motor-driven devices may be considered disadvantageous from a number of standpoints. First, they are relatively expensive to manufacture and calibrate. Second, the electric motor used to drive each device tends to introduce both electrical and audio noise that is either detrimental to the instrument performance or unpleasant to the instrument operator. Third, owing to their size and position within the instrument, such devices are relatively costly to maintain and replace.